1. Field of the Invention
The present invention relates to a cold cathode type electron emitting device which can be applied to a display device, an exposure apparatus, etc., to the method of manufacturing the cold cathode type electron emitting device and to the method of driving the cold cathode type electron emitting device.
2. Description of the Related Art
A cold cathode type electron emitting device having a planar structure has been conventionally known. The electron emitting devices called a surface conduction type or planar type MIM device are such that they are provided with a pair of device electrodes which are formed on a flat insulating substrate and spaced away from each other by a predetermined distance, and also with an electron emitting portion positioned on a thin film which is interposed between these device electrodes. Since these electron emitting devices are simple in structure, they are suited for use in constructing, for example, an electron source array which is constituted by a large number of electron emitting devices formed on the same substrate.
It is now attracting much attention, as one example of the application of such an electron source array, to fabricate a thin planar display. The principle of luminescence thereof is the same as that of CRT, i.e., the electronic excitation of fluorescent substance is utilized. Since this thin planar display is excellent in energy efficiency, it is possible to realize a self-emission type thin planar display of low power consumption, high luminance, and high contrast.
There is known, as an example of the planar type MIM device, a device where a pair of gold electrodes are formed on a substrate and a discontinuous gold film is formed between this pair of gold electrodes. The device of this structure can be manufactured by the following procedures. First of all, a pair of planar gold electrodes are formed on an insulating substrate. Then, a gold thin film having a sufficient thickness for permitting an electrical conduction between these gold electrodes is formed. Thereafter, an electric current is permitted to flow between these gold electrodes to generate Joule heat, thereby causing the gold thin film to fuse and be destroyed, thus cracking the gold thin film and obtaining a discontinuous gold film. The gold film immediately after this discontinuation is high in electrical resistance. The procedures for effecting the discontinuation of a thin film through the application of an electric current is called “B forming (basic forming)”.
The structure obtained in this manner is then subjected to a procedure called “A forming (adsorption assist forming)”. This assist forming is performed by applying a voltage of 20V or less to the device in a very low pressure atmosphere containing hydrocarbons. As a result, due to the action of high electric field generated in the discontinuous film, the electrical resistance of the device is enabled to decrease within a several minutes, thus making it possible to increase the electric current of the device.
It is reported in Pagnia, Int. J. Electronics, 69(1990) 25, and in Pagnia, Int. J. Electronics, 69(1990) 33 that the region between the gold electrodes of the device after the application of the aforementioned A forming is entirely covered with a conductive film. This conductive film is a film containing carbon.
Further, under a condition where a third electrode (anode) is disposed opposite to the device, when electric current flows through the device and at the same time, a positive voltage is applied to the third electrode, current flow between these electrodes can be observed and at the same time, current flow between the device and the third electrode can be observed. If the current flow between these electrodes is defined as an device current and the current flow between the device and the third electrode is defined as an emission current, the ratio of the emission current to the device current (emission efficiency) is extremely small, i.e. 1×10−4% or so (see for example, Pagnia, Phys. Stat. Sol. (a) 108(1988) 11).
On the other hand, the surface conduction type device has a structure which is similar to the aforementioned planar type MIM device. In the example that has been reported (see for example, JP Laid-open Patent Publication (Kokai) No. 11-297192(1999)), the surface conduction type device is formed through a process wherein the device is subjected to a step of the aforementioned “forming” in the same manner as the aforementioned planar type MIM device to thereby form an electrically discontinuous portion in the conductive thin film which has been formed between a pair of electrodes, and then subjected to a step of activation to thereby permit a deposit layer containing carbon to be deposited on the surface of the conductive thin film. Incidentally, in the example described in JP Laid-open Patent Publication (Kokai) No. 11-297192(1999), a voltage is applied between the device and anode to generate plasma, thereby performing the cleaning of the conductive thin film.
An image display device can be fabricated through a procedure wherein a large number of the aforementioned surface conduction type devices are arrayed and phosphors are respectively disposed opposite to each of the devices. The brightness which is one of the important display characteristics of an image display device is dependent on the luminescence intensity of the phosphors, but is also positively correlated with the emission of electric current. Namely, even if the emission efficiency is constant, if the device current is increased, it is possible to increase the emission current, so that if it is desired to increase the emission current, it will be realized by increasing the device current. If it is desired to increase the device current, it will be realized by increasing the size of the device. However, in view of the image resolution, the increase in size of the device is limited, and additionally, there is an upper limit with respect to the density of device current from the viewpoint of the thermal stability of the device.
Incidentally, a voltage is applied also to the aforementioned “cracked portion” which is an electrically discontinuous portion. Therefore, there is also a limitation with regard to the voltage in order to retain a predetermined degree of electric field so as to prevent the cracked portion from generating an electric discharge. This limitation is also effective for limiting the upper limit of the current density.
It will be understood from the foregoing explanation that it is imperative to increase the emission efficiency in order to increase the emission current.